1. The Field of the Invention
The present invention relates to catalysts for use in reforming hydrocarbons. More particularly, the invention relates to reforming catalysts having a controlled coordination structure and methods for manufacturing such catalysts and reforming hydrocarbons.
2. The Relevant Technology
Naphtha is a volatile, flammable liquid mixture of hydrocarbons distilled from petroleum or other fossil fuel sources. Naphtha can be used as a fuel, a solvent, or in making various chemicals. Typically naphtha is a mixture of hydrocarbons that boil between about 65° C. and about 195° C. and is obtained by processing crude oil and optionally heavy oil fractions.
Naphtha reforming is an important refinery process where naphtha is upgraded into more valuable hydrocarbons having a higher octane rating. In reforming, naphtha is heated and fed into a series of reactors loaded with a solid supported metal catalyst. Typically, the catalyst contains platinum and one or more additional metals, which are supported on alumina.
The main goal of the reforming process is to convert the feed into a liquid product stream with a higher octane number. The octane number is a measure of the performance of the hydrocarbons in a gasoline internal combustion engine. Thus, naphtha reforming converts hydrocarbons streams into a reformate product that is more suitable as a gasoline blending stock. The octane number gain during the reforming process varies depending on, among other factors, the original crude and the reforming conditions. Typical octane value increases are in a range between 30 and 70.
The reforming process involves various parallel and consecutive reactions. The reforming process improves octane by increasing the percentage of branched and/or aromatic compounds in the reformate. In addition to the production of gasoline blending stocks, reforming is a very significant source of aromatics (e.g., benzene, toluene, and xylenes, collectively known as BTX), which are used extensively in the chemical and petrochemical industries (e.g., as solvent or starting materials). The reforming process is also the only refinery operation that yields a net production of hydrogen. Hydrogen is extremely valuable because it finds extensive use elsewhere in a refinery, particularly for the upgrading of low quality hydrocarbon streams.
Among the multiple reactions taking place during the reforming operation, some of them are undesirable, at least to some extent. One of these reactions is hydrocracking, which occurs when a hydrocarbon chain is broken apart into two smaller molecules with a net consumption of hydrogen. Hydrocracking in the reforming process can be undesirable because it consumes valuable hydrogen and can produce smaller hydrocarbon fragments outside the range of liquid reformate product (C5+ hydrocarbons). Light gases (C1-C4 hydrocarbons) formed by hydrocracking are not a desired reformate product. Production of C1-C4 hydrocarbons during reforming is considered a loss that reduces the overall yield of the reforming process.
While reforming catalysts are essential to improving octane number, reforming catalysts are also known to catalyze hydrocracking. The catalytic properties of a catalyst are determined by both the type of active components selected, i.e., the elemental composition of the catalyst, and the detailed structure of the dispersed particles, i.e., the atomic scale structure and orientation of the dispersed particles.
Historically, much of the work in the development and optimization of catalysts in general has focused on the selection of the appropriate catalytic components. Prior methods have allowed catalyst developers to control the selection and relative amounts of catalyst components. However, the control of the detailed structure of catalysts, particularly on the atomic scale, has presented a much greater difficulty. However, controlling the atomic scale structure can be as important in the development of effective catalysts as selecting the elemental composition. For example, control of the detailed catalyst crystal structure can relate directly to the selectivity of the catalyst for a particular reaction.
One particularly useful way of defining a preferred catalytic structure is based on the geometry of the surface active sites. Because of thermodynamic considerations, it is normally the case that particles of crystalline materials will expose one or more of a limited number of low-index crystal faces. Common low-index crystal face exposures of metal particles include, for example, the 111, 100, and 110 crystal faces of the common crystal lattices, which include face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close-packed (HCP). Each of these crystal faces has a different arrangement of atoms, and may therefore display different catalytic properties with respect to certain chemical reactions.